I'm a bit rusty on this, but basically Clomid acts to sort of "jump start" your hypothalamus. The hypothalamus tells the pituitary the release FSH and LH which act on the testicles.
When you use AAS, the feedback loop to the hypothalamus gets suppressed or even shut down, depending on the dosages and length of time you're on. The higher the dose, and the longer the time you're on - the greater the suppression. This is what people are talking about when they talk about "shutting down the HPT axis."
HCG will act on the testicles to keep them functioning, the problem is that the hypothalamus still reads everything as ok, so sees no reason to signal the pituitary to send FSH and LH out to stimulate the testicles.
You can use Clomid and HCG to try and avoid suppression of the HPT axis as Big A says, and there are a number of ways to do this.
I've thought shorter cycles (6-8 weeks) and the lowest dosages possible is also a good adjunct to using Clomid and HCG, as you suppress the HPT axis less. Also use anti-estrogens to keep the estrogen down while the body normalizes, especially if you use highly-aromatizable AAS like test, d-bol, anadrol, etc.